This invention relates generally to television receivers and is specifically directed to a system for improving the video display in a microprocessor controlled television receiver.
Keyboard tuning which has been available in color television receivers is now finding its way into cheaper, black and white television receivers. This approach to channel selection is generally faster, more accurate, and offers longer useful lifetimes of the components utilized therein. The selected channel number may be displayed either temporarily following channel selection on a portion of the video display itself, or by means of a continuous, multi-segment, two digit, light emitting diode (LED) display.
With keyboard channel section has come the utilization of microprocessors for controlling such functions as tuning, channel number display, television receiver remote control, and an ever increasing number of television receiver functions. Microprocessors have the advantage of being small, low power consumption devices which are programmable by operator-initiated instructions for executing mathematical algorithms on the data supplied to the microprocessor. The microprocessor is capable of high speed execution of a set of instructions and providing output data during execution which may be used to control various operating modes and parameters of the television receiver. Data instructions are input to and output from the microprocessor at extremely high rates and frequently on the same lines in order to minimize system cost and complexity by limiting the number of pins of the microprocessor.
The digital switching necessary for data transfer to and from the microprocessor unfortunately generates noise which may degrade the video display of the television receiver. In a black and white television receiver which is vertically synchronized with the 60 Hz VAC input power line, this problem may be avoided by driving one light emitting diode (LED) display, such as the units element, during the first half cycle, and a second LED display, e.g., the tens digit, during the second half cycle. Unfortunately this approach will not solve the problem in a color television receiver where the actual vertical rate is 59.94 Hz. Thus, an attempt to synchronize (sync) LED display drive signals with the AC input line will result in the continuous vertical rolling of a noise band appearing in the form of a horizontal speckled line across the faceplate of the cathode ray tube (CRT).
One attempt at reducing this source of video display degradation involves the use of choke coils each coupled in circuit with an LED segment for filtering out this digital switching noise from the video signals displayed on the CRT's faceplate. This approach, however, is undesirable in that it overly complicates LED drive circuitry and substantially increases channel number display system cost. Another approach makes use of a latched channel number display in which a separate integrated circuit is provided with selected channel number information which is latched into this integrated circuit for later transfer out to the channel number digits display. This too is an expensive solution to the problem since fourteen LED segment lines and associated pins on the integrated circuit latch are required. In addition, this approach requires a dedicated integrated circuit which substantially increases system cost.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a system for scanning a channel selector keyboard and driving an LED channel number display by means of a microprocessor in a television receiver without degrading its video display with high frequency digital switching noise.